Risers and offshore systems comprising such riser or risers are well known in the art. Such risers are usually applied for subsea fluid transportation of fluids such as petrochemical products, e.g. from a subsea facility e.g. a production well to an upper facility e.g. a sea surface installation. Examples of well known risers include steel catenary risers (SCR) e.g. as described in WO2011/064591, metallic rigid risers, flexible risers e.g. as described in standard “Recommended Practice for Flexible Pipe”, ANSI/API 17 B, fourth Edition, July 2008, and the standard “Specification for Unbonded Flexible Pipe”, ANSI/API 17J, Third edition, July 2008 as well as any other offshore pipes for fluid transport between facilities arranged at different water levels.
Such risers are often very long in particular when they are arranged for deep water applications e.g. for production at deep water fields. Such riser will therefore usually be subjected to different sea depths, and thereby the different length sections of the riser will be subjected to different conditions such as current, waves and tides.
Furthermore, when the upper facility is a floating unit, the riser will be subjected to large movements, which requires high flexibility of the riser.
In order to alleviate large movements of floating units e.g. floating units of offshore site systems such as FPSO units (floating production, storage and offloading units) or FSO units (floating storage and offloading units), it has for many applications been desired to use flexible pipes in particular for deep water applications. However, for deep water applications such flexible risers are very heavy and must be supported with positive buoyancy modules at several positions along its length so as to distribute the suspended weight between the upper and lower parts of the riser. Such buoyancy arrangement is expensive, difficult to install and to control, and it is therefore generally undesired to rely on such complicated buoyancy arrangements.
In U.S. Pat. No. 6,364,022 a hybrid pipe for deep water is described, comprising a metallic rigid central part having an upper end, which is connected to an upper portion of a flexible pipe of predetermined length and having a lower end, which is connected to a lower portion of the flexible pipe of a length at least equal to the length of the upper portion of the flexible pipe. It is described that the advantage is the riser obtained is rigid for the most part and has the properties of flexible pipes where it leads off from the surface support and where it touches down on the sea bed. Thereby the upper flexible portion takes up all of the weight of the central rigid part and of the lower flexible portion, whereas the latter takes up practically all the dynamic stresses generated by the movements of the surface support.
However, for applications in very dynamic systems, such as where the riser will be subjected to very high dynamic stress e.g. generated by movement of the upper facility or due to weather conditions and/or tides, there is a need for improved riser systems compared to the above described hybrid riser, since in practice the metallic rigid central part provides an undesired stiffness to the system, which results in that the flexible pipe sections will be subjected to excessive and often uncontrolled stress immediately adjacent to their respective connection to the metallic rigid central part.